Due to the increased communication traffic of recent years, adoption of the DP-16QAM (quadrature amplitude modulation) method (for example, refer to Non-Patent Document 2) in the backbone network of the optical communication system is being considered for the next generation 400 Gbps class transmission to replace the DP-QPSK (dual polarization-quadrature phase shift keying) method (for example, refer to Non-Patent Document 1). The DP-QPSK method is a modulation method that is adopted as a standard for 100 Gbps class transmission. The DP-16QAM method is a modulation method with higher frequency utilization efficiency than the DP-QPSK method.
In the DP-16QAM, each polarized wave is independently modulated by 16QAM for the X polarized wave and Y polarized wave. On the reception side, a modulated signal is input to a coherent receiver and then digitized by an ADC (analog to digital converter). Then, polarized wave separation is performed on the signal by means of digital signal processing, and symbol determination is performed on the signal as an independent signal.
As a method for further increasing the transmission capacity, further multi-leveling modulation of signals of the DP-64QAM method (for example, refer to Non-Patent Document 2) or the like, the frequency utilization efficiency of which is higher than DP-16QAM in 1 Tbps class transmission, is being considered. However, multi-leveling modulation of signals would cause the minimum Euclidean distance between symbols to reduce, cause noise resistance properties to decrease, and limit transmission distance.
In recent years, with respect to the reduction in the minimum Euclidean distance accompanying the improvement of the frequency utilization efficiency, there has been proposed a modulation method by means of signal point arrangement in an N-dimension space (for example, refer to Non-Patent Document 3). The modulation method expands the design dimension of signal point arrangement to polarization, time, and wavelength direction, which had been treated as independent dimensions in the past, and expands the minimum Euclidean distance by returning the relationship between the frequency utilization efficiency and the minimum Euclidean distance to the overview of the sphere packing problem in the N-dimensional space.